1. Field of the Invention
The present invention relates to a differential amplifier circuit utilizing thin film transistors, a buffer amplifier utilizing the same and a displays such as liquid crystal displays and electro-luminescence display utilizing the same.
2. Description of the Prior Art
Conventional semiconductors used for liquid crystal displays utilizing thin film transistors include amorphous silicon, high temperature polysilicon formed using a maximum processing temperature of 1000° C. or the like and low temperature polysilicon formed using a maximum processing temperature of 600° C. or less.
Recently, there are displays which utilize medium temperature (temperatures between the high and low temperatures) polysilicon formed using a maximum processing temperature in the range from 600° C. to 1000° C. and displays which utilize a mixture of amorphous silicon and low temperature polysilicon.
FIG. 5A shows a liquid crystal display. Pixels 1001 are arranged in the form of a grid and are each connected to a data line 1005 and gate line 1004. A signal from a terminal 1021 such as a computer is converted by a D-A converter 1020 from a digital signal to an analog signal which is sent to a video signal line 1022. When the terminal 1021 is a television set which outputs an analog signal by itself, the D-A converter 1020 is not provided. The video signal line 1022 is connected to the source or drain of a switch transistor 1003 connected to each data line, and the switch transistor 1003 is opened when an open signal from a horizontal scan circuit 1010 is supplied to a switch gate line 1012 to supply a video signal to the data line 1005.
The video signal supplied to the data line 1005 is supplied through a buffer amplifier 1002 to the pixel 1001. The area of a pixel is shown in FIG. 5B. The data line 1005 is connected to the source or drain of the pixel switch transistor, and the source or drain which is not connected to the data line is connected to a liquid crystal 1050 and a storage capacity 1040.
The gate of the pixel transistor is connected to a gate line 1004 which is connected to a vertical scan circuit 1011.
The semiconductor used for this pixel transistor is any of amorphous silicon, low temperature polysilicon, medium temperature polysilicon, high temperature polysilicon and a mixture of amorphous silicon and low temperature polysilicon as described above.
Referring to a liquid crystal display utilizing amorphous silicon as a pixel transistor, amorphous silicon can not be used as a semiconductor for a buffer amplifier, switch transistor, horizontal scan circuit, vertical scan circuit or D-A converter because the field effect mobility of amorphous silicon is too small.
Therefore, the above-described circuits employ a monolithic IC utilizing a crystalline semiconductor such as single crystal silicon.
This results in a problem in that the surface area of regions on a liquid crystal display other than the display portion becomes large.
Further, cost reduction can not be achieved because of a high IC cost because a monolithic IC utilizing a crystalline semiconductor is used.
Under such circumstances, the use of polysilicon has been proposed. In the case of a liquid crystal display utilizing a low temperature polysilicon, medium temperature polysilicon, high temperature polysilicon or a mixture of amorphous silicon and low temperature polysilicon, since the field effect mobility of such a material is 10 to 100 times greater than that of amorphous silicon, it can be used as a semiconductor for the buffer amplifier, switch transistor, horizontal scan circuit and vertical scan circuit. A bipolar transistor or the like utilizing single crystal silicon semiconductor is required for a D-A converter which converts a digital signal into an analog signal in a very short period of time.
In the case of a liquid crystal display utilizing a low temperature polysilicon, medium temperature polysilicon, high temperature polysilicon or a mixture of amorphous silicon and low temperature polysilicon, since the pixel switches, buffer amplifier, switch transistors, horizontal scan circuit and vertical scan circuit can be formed on a substrate using polysilicon, it is possible to reduce the surface area of regions of the liquid crystal display other than the display portion.
There is another advantage in that cost reduction can be achieve in an amount corresponding to a reduction in the IC cost because no monolithic IC utilizing a crystalline semiconductor is used.
FIGS. 6A and 6B show conventional buffer amplifiers which can be formed using polysilicon. FIG. 6A shows a source follower formed using an n-channel type thin film transistor 1101 and a load resistor 1102, which is a buffer amplifier having a gain of substantially 1. 1103 designates an input terminal; 1106 designates an output terminal; 1104 designates a positive power source; and 1105 designates a positive power source.
FIG. 6B shows a source follower formed using a p-channel type thin film transistor 1111 and a load resistor 1112, which is a buffer amplifier having a gain of substantially 1. 1113 designates an input terminal; 1116 designates an output terminal; 1114 designates a positive power source; and 1115 designates a negative power source.
These buffer amplifier can be very simply formed because they are formed using only one transistor. However, since the transistors have great variation and greatly fluctuate in characteristics in response to temperature changes, they become inoperable as a buffer amplifier for an increased number of tones because such variation hides some tones, although they work properly for a liquid crystal display having a small number of display tones.
Referring to tones, when a liquid crystal display is driven by 5 V, the voltage width for one tone where there are eight tone in total is 5 V/8 that is 625 mv. The voltage width will be 313 mV, 156 mV, 78 mV, 39 mV and 20 mv where there are 16, 32, 64, 128 and 256 tones, respectively.
The number of tones that one transistor can provide is limited to the range from 16 to 32 tones because of changes in characteristics and variation from one transistor to another.
Further, even if a differential circuit is formed using two transistors, there is a problem inherent to polysilicon which has hindered variation in characteristics from being improved.
FIGS. 7A and 7B illustrate the problem inherent to polysilicon. FIG. 7A is a plan view of an insulated gate type polysilicon thin film transistor (hereinafter referred to as polysilicon thin film transistor) comprised of a gate electrode 1202, polysilicon 1201, a source electrode 1203, source contacts 1205, a drain electrode 1204 and drain contacts 1206. The polysilicon 1201 includes grain boundaries 1210, 1211 and 1212. The grain boundaries vary depending on the conditions under which the polysilicon has been crystallized, crystal nuclei, and their positions in the substrate rather than being formed uniformly.
FIG. 7B shows a sectional view taken along the line X-X′ wherein the polysilicon 1201, the gate electrode 1202, a layer insulation film 1208, the source electrode 1203, the source contacts 1205, the drain electrode 1204 and the drain contacts 1206 are shown on a substrate 1200. The grain boundaries 1210, 1211 and 1212 are formed in the form of cuts into the section as illustrated.
Carriers flowing from the source to the drain traverse the grain boundaries.
The problem is not significant when there is only one polysilicon thin film transistor. However, when a buffer amplifier or the like is formed by a plurality of transistors, the characteristics of the individual transistors vary slightly because the grain boundaries in the individual transistors vary. This is unsuitable for use in thin film transistor circuits such as differential amplifier circuits wherein transistors should have the same characteristics, and it has been only possible to configure buffer amplifiers having 64 tones at most.
FIG. 9 shows crystallization utilizing a laser which is a method of forming low temperature polysilicon. FIG. 9 is a plan view of island-shaped semiconductors 1401, 1402, 1403, etc. provided on a substrate. The island-shaped semiconductors 1401, 1402, 1403, etc. are crystallized by a laser by irradiating areas 1410, 1411, 1412 and 1413 with the laser sequentially.
In the case of liquid crystal displays, the diagonal size of substrates is significantly greater than 1 inch and is normally 3 inches or more, and the diagonal size can be 8 inches or more where a multiplicity of liquid crystal displays are taken out from one substrate. Since it is thus impossible for a laser to irradiate the entire surface of a substrate to crystallize it at a time, laser irradiation areas are sequentially scanned as shown in FIG. 9 to crystallize the semiconductor on the entire surface of a substrate.
In this case, the laser irradiation areas may overlap each other, resulting in increased variation of crystallization of the semiconductor in such a region.
While an excimer laser is normally used, an excimer laser can not be normally used for fabricating polysilicon for thin film transistor circuits such as differential amplifier circuits wherein transistors must have the same characteristics because it realizes an excimer state using an inert gas such as argon or xenon and a halogen gas such as fluorine or chlorine to produce ultraviolet light having high energy and, as a result, there is variation of irradiation energy as great as 5% even in laser irradiation areas. Thus, it has been possible to configure only buffer amplifiers having 32 to 64 tones at most.
The method of fabricating polysilicon using a metal catalyst disclosed in Japanese unexamined patent publication No. H8-335152 associated with the patent application by the present applicant allows crystals to grown in the same direction. This process is shown in FIG. 8 which is a plan view of a polysilicon thin film transistor comprising a gate electrode 1302, polysilicon 1301, a source electrode 1303, source contacts 1305, a drain electrode 1304 and drain contacts 1306. The polysilicon 1301 includes acicular or columnar grains 1330. This is because there is a metal catalyst 1340 for promoting crystal growth which serves as crystal nuclei from which crystals grow. In a position symmetrical about the metal catalyst 1340, there is provided another polysilicon thin film transistor 1350 which includes acicular or columnar grains 1331.
Polysilicon thin film transistors formed according to this method have transistor characteristics much higher than those of polysilicon thin film transistors utilizing polysilicon crystallized by other processes such as crystallization wherein only heat is applied, laser crystallization and crystallization performed after forming a metal catalyst on the entire surface of a substrate. For example, while a drive signal has had a frequency on the order of only several MHz according to the prior art, the thin film transistors described above can be driven at frequencies of several tens MHz, several hundred MHz or more using a voltage in the range from 3.3 V to 5 V and can be operated at 1 GHz on the ring oscillator level and at 100 MHz on the shift register level.
However, even such thin film transistors having high characteristics formed utilizing a metal catalyst can not be currently used as they are for a differential amplifier circuit used as a buffer amplifier using two transistors to provide 256 tones, and they have been able to provide only 64 to 128 tones at most.
As described above, in a liquid crystal display incorporating a buffer amplifier using polysilicon thin film transistors, it has been difficult to form a buffer amplifier using a differential amplifier circuit having two transistors because the individual transistors have varied significantly depending on the method of fabrication used for the polysilicon.
In the case of thermally crystallized polysilicon, there is only provided buffer amplifiers having 64 tones at most because the grain boundaries are formed irregularly.
In the case of polysilicon crystallized using a laser, there is only provided buffer amplifiers having 32 to 64 tones at most depending on the degree of the overlapping of laser irradiation and instability of laser energy.
In the case of polysilicon utilizing a metal catalyst, there is provided buffer amplifiers having 128 tones at most.
According to the prior art, it has not been possible to fabricate a buffer amplifier constituted by a differential amplifier circuit utilizing a plurality of polysilicon thin film transistors formed from polysilicon materials produced according to different processes.